When I was just starting out in my painting career, I would paint on anything, just to get a feel for different types of surfaces. Canvas panels; used canvas that I'd paint over; even thin wood panels. My financial situation also came into play, as I really couldn't afford anything but the most rudimentary type of canvas.
Cheap canvas is okay for learning on, but these are it's drawbacks: It is extremely coarse, making it very difficult to create a crisp, straight brush stroke. Both hard and soft edges are more frustrating. It is also tricky at best to blend colors on, as the paint collects in the rough texture, thus making the paint hard to mix on the canvas. (Blending is a refined method of mixing colors on the canvas).
As soon as you can afford it, you should jump to a finer quality canvas that is smoother and better primed. With this one improvement, you'll find you can manipulate the paint more deftly, and your brushes will even perform better. But even this better quality canvas is not as good for realism painting or portraits as my favorite: linen canvas.
Ahh. How can I describe the joy of painting on smooth, quality linen? It's the difference between sleeping on rocks as opposed to sleeping on feathers. Suddenly you feel like a better painter. You have a steadier hand; your edges are ultra sharp where you want them to be, and softer than soft where you wish the object to recede.
I wish I could justify buying the most expensive linen canvas, but it is much more pricey than the mid grade. So I purchase the lower priced linen and couldn't be happier.
Maybe next year I'll graduate to the super quality. Who knows what that'll do for my painting!